1. Field of the Invention
The invention relates to a solenoid valve, in particular for controlling the brake pressure in a wheel brake of a slip-controllable hydraulic brake system of a motor vehicle.
2. Description of the Prior Art
Solenoid valves of this kind are already known, for example, from the applicant's prior patent application with the filing number DE 10 2007 053 134.8.
In the following, slip-controllable hydraulic brake systems of motor vehicles are understood to be antilock brake systems (ABS), traction control systems (TCS), or electronic stability program systems (ESP). In these control systems, it is possible to use solenoid valves to control or regulate the brake pressure of at least one wheel brake, e.g. as a function of the slip ratios occurring in the respective wheel. These solenoid valves are triggered by means of an electronic control unit that evaluates sensor signals to accomplish this.
The solenoid valves used in these control systems are of two basic known types: either solenoid valves are used, which are embodied in the form of switching valves that can be switched into two discrete positions (starting position and switched position) or are embodied in the form of continuous control valves (=proportional valves) that can assume any of a multitude of intermediate positions between an open position and a closed position. The invention can be used equally with switching valves and continuous control valves.
Among other essential components, the solenoid valves described in DE 10 2007 053 134.8 have a valve insert in which at least one inlet and one outlet, respectively, are embodied. For cost reasons, the valve insert is manufactured by being rolled from a sheet metal strip, the two ends of which rest against each other virtually without gaps. The reason for this is that the presence of a possible gap would constitute an undesirable obstacle within the magnetic circuit of the solenoid valve and should therefore be avoided.
To control a hydraulic connection between the inlet and outlet, a valve element is provided, which constitutes a control cross-section. In the above-cited prior art, the control cross-section is embodied in the form of a valve seat. This valve seat can be controlled by means of a tappet that is accommodated for this purpose in sliding fashion in the valve insert. The tappet is actuated indirectly by means of an armature that is movably accommodated in an armature chamber. The armature chamber is delimited by a capsule that is closed at one end and is affixed to the valve insert.
A valve insert equipped with a capsule; an armature, a tappet, and a valve element constitutes a so-called valve cartridge. An annularly embodied, electronically triggerable coil is slid onto this valve cartridge. When this coil is triggered, the armature is subjected to magnetic forces that act in the closing direction of the solenoid valve. These closing forces act in opposition to hydraulic forces and mechanical spring forces, which load the armature in an opening direction, toward a starting position. As soon as the magnetic forces exceed these opening forces, the armature executes a working stroke. The armature movement is transmitted to the tappet, which therefore closes the valve seat. When the electrical triggering of the coil ceases, the spring force moves the tappet back and the solenoid valve once again assumes its open, through-flow position.
Under operating conditions, the interior of a solenoid valve designed in this way is completely filled with pressure fluid to be controlled. However, gas (air) can be dissolved in this pressure fluid. Under certain operating conditions, this gas can outgas from the pressure fluid and form gas bubbles. These tend to collect in the armature chamber. Particularly with continuous control valves, gas bubbles can cause oscillations of the tappet, which are audibly perceptible in the form of noise and cause a disadvantageous change in the operating behavior of the solenoid valve. In the known solenoid valves, there is in fact a first hydraulically effective connection between the armature chamber and the outlet via the guidance of the tappet in the valve insert, but this connection ends in a fashion similar to a blind hole so that no pressure flow can occur in the armature chamber.
Consequently, in solenoid valves according to the prior art, there is the risk that once the gas bubbles have collected in the armature chamber, they can only be removed from the valve cartridge of the solenoid valve with difficulty.